1. Field of the Invention
The present invention relates to open pore, microporous ceramic materials and their method of manufacture.
2. Description of Related Art
Porous materials play a particularly important role in a number of chemical processing industries and applications. Separations based on membranes are critical in such fields as chemical recovery, purification and dehumidification. Porous oxides (e.g., clays, silica, alumina and zeolite) are the materials of choice as catalysts or catalyst supports in chemical and petroleum processing reactions such as hydrocracking, hydrodesulfurization, reforming, and polymerization.
With respect to membrane technology, inorganic membranes offer a number of advantages over polymeric membranes which are typically limited to uses at temperatures below about 250.degree. C. These include: i) higher operating temperatures, ii) greater structural integrity and hence the ability to withstand higher pressure differentials and backflushing, and iii) improved resistance to corrosion. Porous oxide (e.g., aluminum oxide) and carbon membranes offer some of these characteristics, but advanced materials are still required for improved strength, toughness, structural integrity, temperature stability, water and oxygen resistance, thermal shock resistance, molecular selectivity to small molecules and gases, and high flux.
Similar considerations apply to clay and metal oxide type catalysts or catalyst supports, particularly as relates to stability and thermal shock resistance at temperatures above about 500.degree. C.
Ceramic materials of the Si--C, Si--N, Si--C--N, Si--B--C, Si--BN, Al--N, Si--Al--N, B--Al--N and related types appear to offer many of the properties set forth above. However, the solgel synthesis methods typically used to prepare porous oxide membranes or catalyst supports are incompatible with the preparation of ceramics of the type described above because of the need to use water in their preparation. Sintering or reactive sintering of these ceramics likewise produces materials with pore sizes of from about 0.1 to about 1000 microns which are non-uniform and generally too large for effective molecular separation and other uses described above. Chemical vapor deposition can produce microporous ceramic layers, but this tends to be an expensive, high temperature process with limited ability to tailor complex ceramic compositions.
Recently, researchers have discovered improved methods for preparing ceramics using ceramic precursors as starting materials. A ceramic precursor is a material, a chemical compound, oligomer or polymer, which upon pyrolysis in an inert atmosphere and at high temperatures, e.g., above about 200-400.degree. C., preferably above 700-1000.degree. C., will undergo cleavage of chemical bonds liberating such species as hydrogen, organic compounds and the like, depending upon the maximum pyrolysis temperature. The resulting decomposition product is typically an amorphous ceramic containing Si--C bonds (silicon carbide), Si--N bonds (silicon nitride) or other bond structures which will vary as a function of the identity of the ceramic precursor, e.g., Si--C--N, Si--N--B, B--N, Al--N and other bond structures, as well as combinations of these structures. Crystallization of these amorphous ceramic products usually requires even higher temperatures in the range of 1200-1600.degree. C.
The pyrolysis of various ceramic precursors, e.g., polycarbosilanes, polysilanes, polycarbosiloxanes, polysilazanes, and like materials at temperatures of 1200.degree. C. and higher to produce ceramic products, e.g., silicon carbide and/or silicon nitride, is disclosed, for example, in M. Peuckert et al, "Ceramics from Organometallic Polymers", Adv. Mater. 2, 398-404 (1990).
During pyrolysis, preceramic precursors such as described above liberate various gaseous decomposition species such as hydrogen and organic compounds, including methane, higher molecular weight hydrocarbon molecules, and lower molecular weight precursor fragments. These gases tend to coalesce within the preceramic matrix as they form, resulting in a bulking or swelling to form a voluminous mass of low bulk density. These entrained gases can also lead to the formation of smaller gas bubbles within the developing ceramic mass as the preceramic precursor crosslinks and hardens, resulting in a reduced density ceramic having a mesoporous or macroporous closed-cell structure, without development of a significant amount of open celled micropores.
Where dense, non-porous ceramic materials are sought using ceramic precursors yielding high gas volumes, it is often necessary to conduct the pyrolysis over a very long period of time with very gradual incremental temperature increases and/or under vacuum to assist in removal of these gaseous species at temperatures where they are formed.